For many diagnostic blood tests, it is required or preferable to eliminate red blood cells, white blood cells, and platelets from the patient's blood so that only the blood plasma remains for analysis. In laboratories, blood is typically segregated by centrifugation, and the plasma is siphoned and transported to the analysis instrumentation. This requires specialized instrumentation and significant energy consumption. An emerging objective in point-of-care diagnostics is to perform analysis of a patient's blood using a minimal quantity blood. This has led to the development of microfluidic devices for blood separation and blood testing. In microfluidic devices, blood may be propelled through a filter or separator and the plasma filtrate pumped into analysis chambers. Separation techniques include ultrasonic separation, dielectrophoretic separation, and mechanical size selection. These are active separation techniques which require complicated power systems and plasma transportation channels to move the separated plasma into an analysis region.
Prior passive microfluidic blood testing devices have utilized capillary flow to transport whole blood across an analysis zone. Using the whole blood, rather than just the plasma, has negatively impacted the capillary flow and, in some cases, whole blood is not suitable for analysis. Vacuum flow has also been used; however, vacuum flow devices require long-term storage of a low pressure reservoir. This complicates the fabrication process and entails a higher material cost, which makes a vacuum flow device less practical for wide-scale use.